Device for the induction heating of a workpiece
The present invention relates to methods and devices for magnetically heating a workpiece consisting at least partly of metal More specifically, the invention is concerned with the preheating of moulding or press tools, the separation of metal from a polymer, the production of composite materials, and the hardening of adhesive between two metal members. 
     For the preheating of press tools, which is necessary before the tool is operated, use is made at present of hot-air furnaces in which the tool is heated to about 450°. This takes about 10 hours, which is far too long for an efficient production. Furthermore, it is difficult in a furnace, to heat the press tool to a uniform temperature across the entire tool cross-section. 
     Hitherto, a number of techniques have been utilised to separate polymeric materials, such as rubber and plastic from metal. One technique involved burning-off of the polymer, which has caused emissions hazardous to the environment, and structural changes and stress in the metal. DE 2,900,655 indicates a different technique involving the use of an induction coil. An object, for example a vehicle tire, is positioned within the induction coil which is activated by a high-frequency voltage to generate a magnetic field heating the metal in the vehicle tire to such an extent that the polymer can be detached from the metal. Generating high-frequency voltage is expensive and cannot be utilised for all types of objects. If the object has projecting metal parts, a so-called point effect is obtained which means that the projecting metal part is heated to red heat. Furthermore, the high-frequency technique is restricted to relatively thin objects since the penetration depth in metal is slight when use is made of high frequency, for example 1 to 16 kHz. 
     A first object of the invention is to achieve rapid and uniform heating of a workpiece, without having to rely on the shape and thickness of the object for the desired result. PG,3 
     A second object of the invention is to provide an inexpensive and uncomplicated heating device adapted to utilise the existing electric mains. 
     A third object is to provide a device which, upon separation, gives clean and faultless final products, i.e. metal parts which are clean from polymeric material and which have not been affected by structural changes and stress, as well as polymeric material which is not heated to temperatures at which it is destroyed, the final products being reusable. 
     A fourth object of the invention is to provide an energy-saving heating method by which the losses are restricted to the heat developed in the coils, and to losses in the core. 
     These objects are achieved by devices and methods having the features defined in the characterising clauses of the appended claims. 
    
    
     Embodiments of the invention will be described in the more detail below, reference being had to the accompanying drawings. 
     FIG. 1 shows a first embodiment of the invention, in which a movable core pole is displaced upwardly to produce an air gap adapted to accommodate a workpiece. 
     FIG. 2 shows a second embodiment in which the workpiece is a secondary winding of the circuit. 
     FIG. 3 shows a third embodiment adapted to separate polymeric material from a metal container or the like. 
    
    
     FIG. 1 illustrates a core 1 of transformer sheet steel. The core has a stationary pole 2 and a movable pole 3. The pole 3 is movable into engagement with the pole 2 to form a short-circuited core. In the embodiment shown in FIG. 1, the movable pole 3 is displaced upwardly to form an air gap between itself and the stationary core 2. A first coil 4 is positioned around the movable pole and a second coil 5 is positioned around the stationary pole. A third coil 6 is positioned around the air gap between the first and the second coil. The coils 4, 5 and 6 are connected in series via lines 9, 10, 11 and 12 and connected to an alternating voltage source 7 which is a single-phase source utilizing the zero conductor of the electric mains and gives, for example, 220 V at 50 Hz. Naturally, also two or three phases may be utilised. In one of its aspects, the invention aims at providing adequate heating by means of the normal mains voltage and frequency. 
     The poles 2 and 3 have been designed to retain the workpiece during heating, and therefore there is no need of special means for holding the workpiece during heating 
     The workpiece consists of metal coated with a polymeric material, such as rubber. The workpiece may also be a press or moulding tool. In FIG. 1, the workpiece extends beyond the core, but also smaller workpieces of a width below the cross-sectional area of the core can be heated in a device according to the invention. One example of such a workpiece is a ball bearing. The workpiece may have at least one metal portion bridging the air gap and short-circuiting the core 1. The device is operable also if short-circuiting is prevented by a layer of polymeric material on the workpiece. It should be noted, however, that the losses increase with the thickness of the said layer. 
     Upon application of a voltage, the coils generate a magnetic field which is conducted in the circuit formed by the core 1 and the workpiece 8 and also occurs outside the core. The magnetic field conducted in the core passes through the central metal portion of the workpiece 8 and heats it. The magnetic field outside the core will heat the peripheral metal portions of the workpiece by induction. By using a relatively low frequency in the range 16-400 Hz, adequate penetration depth in the metal is obtainable, and so the metal is uniformly heated. 
     Upon separation of polymeric material and metal, the heating of the metal causes a chemical degradation of the layer of the polymeric material in contact with the metal. 
     The frequency can be varied in dependence on the thickness of the workpiece If a penetration depth of about 2 cm is desired, the frequency is set at about 25 Hz. For heating thin-walled workpieces, the frequency may be for example 200 Hz. 
     In FIG. 1, the coils are connected in series, but can also be connected in parallel, provided that their magnetic fields cooperate to heat the workpiece. 
     At least the first coil 4 can be raised and lowered to control the heating of the workpiece. A microprocessor is preferably used for controlling the movement of the coil 4 and the movable pole 3. If desired, also the remaining coils can be made vertically movable. In some cases, the coil 6 may be omitted, and then the magnetic field generated by the coils 4 and 5 is sufficient to heat both the peripheral portions and the central portion of the workpiece. On the other hand, the coil 6 assists in concentrating the magnetic field outside the core to the workpiece peripheral portions. 
     Heating a workpiece to 450° C. in a conventional hot-air furnace would take 10 hours, but with the new technique according to the present invention the heating takes but a fraction of this time, for example 15 min. The new technique gives a temperature which differs by not more than ±5° C. throughout the cross-sectional area of the workpiece. 
     In FIG. 2, a workpiece in the form of a ring 13 is heated. In this embodiment, the movable pole 3 has been lowered to engage the stationary pole 2. Upon activation of the cores, an electrical current is produced in the workpiece 13 which assists in heating the workpiece. As in the embodiment according to FIG. 1, the magnetic field from the coils 4. 5 and 6 induces heat within the workpiece. 
     FIG. 3 shows a device for removing polymeric material from a large workpiece, such as a metal tank. The device comprises a U core 14, the legs of which are provided each with one coil 17 and 18, respectively. The coils are connected in series by means of lines 19, 20 and 21 and connected to a voltage source 7 of the type mentioned above. The device is applied to the tank 22, and the magnetic field generated upon activation of the coils is conducted through the U core 14 and the tank region between the legs 15 and 16 which is heated to the desired temperature by controlling the applied voltage, the frequency and the time during which voltage is applied When the region has been heated, the device is moved to another region of the tank and the polymeric material of the area first heated simply is peeled off. 
     In the embodiments described above, a short-circuited core has been used. In some cases, it is also possible to utilise only the poles 2 and 3 of the core and to dispense with the remaining core part, although a higher current consumption and higher losses are then obtained